Layered porous film, and non-aqueous electrolyte secondary battery

ABSTRACT

A laminated porous film includes a porous base material layer containing polyolefin as a main component; a filler layer containing inorganic particles as a main component; and a resin layer containing resin particles as a main component, the resin particles showing an endothermic curve satisfying conditions (1) and (2) below, the endothermic curve being obtained by differential scanning calorimetry. Condition (1): a temperature at which DDSC is not less than 0.10 mW/min/mg is not less than 70° C. Condition (2): endothermic amount calculated from an area of the endothermic curve in a range of not less than 50° C. and not more than 70° C. is not less than −20.0 J/g.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of co-pending U.S. patent applicationSer. No. 15/302,057 filed Oct. 5, 2016 which was a Section 371 ofInternational Application No. PCT/JP2015/061299, filed Apr. 6, 2015,which was published in the Japanese language on Oct. 15, 2015, underInternational Publication No. WO 2015/156410 A1, and the disclosures ofwhich are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to (i) a laminated porous film and (ii) anonaqueous electrolyte secondary battery including the laminated porousfilm.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries, particularly lithium ionsecondary batteries, have high energy density, and are therefore widelyused as batteries for personal computers, mobile phones, mobileinformation terminals, and the like. In nonaqueous electrolyte secondarybatteries typified by such lithium ion secondary batteries, a separatoris ordinarily provided between a cathode and an anode.

Nonaqueous electrolyte secondary batteries, typified by lithium ionsecondary batteries, have high energy density. Therefore, in a casewhere an internal short circuit and/or an external short circuit is/arecaused by, for example, breakage of a nonaqueous electrolyte secondarybattery or breakage of a device using the nonaqueous electrolytesecondary battery, a high current flows so as to cause intense heat tobe generated. This has created demands that nonaqueous electrolytesecondary batteries should prevent greater than a certain level of heatgeneration to ensure a high level of safety.

Safety of a nonaqueous electrolyte secondary battery is typicallyensured by imparting to the nonaqueous electrolyte secondary battery ashutdown function, that is, a function of, in a case where there hasbeen abnormal heat generation, blocking passage of ions between thecathode and the anode with use of a separator to prevent further heatgeneration. In order to impart a shutdown function to a separator, it ispossible to use, as the separator, a porous film made of a material thatmelts in a case where there is abnormal heat generation. Specifically,according to a battery using such a separator, a porous film melts so asto be non-porous in a case where there has been abnormal heatgeneration. This blocks passage of ions, and therefore restricts anyfurther heat generation.

Examples of a separator having such a shutdown function encompass aporous film made of polyolefin. A separator, which is made of thepolyolefin porous film, melts so as to be non-porous in a case wherethere has been abnormal heat generation in a battery. This blocks (shutsdown) passage of ions, and therefore restricts further heat generation.However, in a case where, for example, heat generation is intense,thermal shrinkage may occur to the separator, which is made of thepolyolefin porous film. This may cause a cathode and an anode to comeinto direct contact, and therefore poses a risk of causing a shortcircuit. According to a separator which is made of a polyolefin porousfilm, shape stability at a high temperature is thus insufficient.Therefore, it is sometimes not possible to restrict abnormal heatgeneration which is caused by a short circuit.

As a separator whose shrinkage at a high temperature is restricted tohave excellent shape stability, there has been proposed a separatorwhich includes a porous base material layer containing polyolefin as amain component, the porous base material layer having (i) a fillerlayer, provided on one surface thereof, which contains an inorganicfiller as a main component and (ii) a resin layer, provided on the othersurface thereof, which contains resin particles as a main component, theresin particles having a melting point of 100° C. to 130° C. (see PatentLiterature 1). Patent Literature 1 discloses that (i) the separatorincludes the resin layer so that, before a thermal shrinkage temperatureof the porous base material layer is reached, the resin particles meltso as to cause the porous base material layer to be shaped into anon-porous film and (ii) the separator includes the filler layer sothat, even in a case where the thermal shrinkage temperature of theporous base material layer is reached, the presence of the filler layerprevents a short circuit from occurring between electrodes.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication, Tokukai, No. 2011-198532

SUMMARY OF INVENTION Technical Problem

However, a separator is also required to have excellent ionpermeability. According to Patent Literature 1, there is no evaluationof specific ion permeability of the separator having a multilayerstructure by laminating the filler layer and the resin layer on theporous base material layer. There is therefore room for improvement inion permeability.

Furthermore, there is a case where a separator is exposed under a hightemperature environment when the separator is produced or used. Asdescribed later, there is a possibility that a separator having amultilayer structure decreases its ion permeability when exposed under ahigh temperature environment.

In production of the separator having a multilayer structure, a fillerlayer or a resin layer is formed by applying, on a porous base materiallayer whose main component is normally polyolefin, a coating solutioncontaining a component to form the filler layer or the resin layer, anddrying a coating film obtained and removing a medium. There is a casewhere the drying is performed at a higher temperature in order toincrease productivity. It was found that drying at a high temperaturemay result in lower ion permeability of a separator obtained.

There is a case where in a step of producing a nonaqueous electrolytesecondary battery, a separator is exposed under a high temperatureenvironment. The step of producing a nonaqueous electrolyte secondarybattery normally includes a heating and drying step and a thermalpressing step. In these steps, the separator is exposed under a hightemperature environment. It was found that the separator having gonethrough these steps may decrease its ion permeability.

There is a case where a nonaqueous electrolyte secondary battery is usedunder a high temperature environment. In that case, a separator isexposed under a high temperature environment. For example, in a casewhere a nonaqueous electrolyte secondary battery is used in an electricvehicle, the nonaqueous electrolyte secondary battery is normallyprovided in a bonnet, and an inside of the bonnet under the blazing sunis a high temperature environment. It was found that when the nonaqueouselectrolyte secondary battery is used under a high temperatureenvironment as above, ion permeability of a separator decreases.

It is an object of the present invention to provide a laminated porousfilm which has excellent ion permeability, which maintains the excellention permeability even when exposed under a high temperature environment,and which is suitable as a nonaqueous electrolyte secondary batteryseparator.

Solution to Problem

The inventors of the present invention carried out diligent research inorder to attain the object, and, as a result, made the presentinvention.

Specifically, the present invention is directed to the following matters<1> through <13>.

<1> A laminated porous film including: a porous base material layercontaining polyolefin as a main component; a filler layer containinginorganic particles as a main component; and a resin layer containingresin particles as a main component, the resin particles showing anendothermic curve satisfying conditions (1) and (2) below, theendothermic curve being obtained by differential scanning calorimetry.

Condition (1): a temperature at which DDSC is not less than 0.10mW/min/mg is not less than 70° C.Condition (2): endothermic amount calculated from an area of theendothermic curve in a range of not less than 50° C. and not more than70° C. is not less than −20.0 J/g.

<2> The laminated porous film as set forth in the above <1>, wherein theresin particles are Fischer-Tropsch wax.

<3> The laminated porous film as set forth in the above <1> or <2>,wherein: the filler layer is provided on one surface of the porous basematerial layer; and the resin layer is provided on the other surface ofthe porous base material layer.

<4> The laminated porous film as set forth in any one of the above <1>through <3>, wherein: a ratio of a weight per unit area of the fillerlayer to a weight per unit area of the porous base material layer is 0.2to 3.0; and a ratio of a weight per unit area of the resin layer to aweight per unit area of the porous base material layer is 0.1 to 2.0.

<5> The laminated porous film as set forth in any one of the above <1>through <4>, wherein the inorganic particles are at least one selectedfrom the group consisting of alumina, boehmite, silica, and titania.

<6> The laminated porous film as set forth in any one of the above <1>through <5>, wherein the inorganic particles are α-alumina.

<7> The laminated porous film as set forth in any one of the above <1>through <6>, wherein the filler layer contains an organic binder.

<8> The laminated porous film as set forth in the above <7>, wherein theorganic binder is a water-soluble polymer.

<9> The laminated porous film as set forth in the above <8>, wherein thewater-soluble polymer is at least one selected from the group consistingof carboxymethyl cellulose, alkyl cellulose, hydroxyalkyl cellulose,starch, polyvinyl alcohol, acrylic acid, and alginic acid.

<10> The laminated porous film as set forth in any one of the above <1>through <9>, wherein the resin layer contains an organic binder.

<11> The laminated porous film as set forth in the above <10>, whereinthe organic binder is a water-insoluble polymer.

<12> The laminated porous film as set forth in the above <11>, whereinthe water-insoluble polymer is at least one selected from the groupconsisting of an ethylene-vinyl acetate copolymer, an ethylene-acrylicacid ester copolymer, a fluorine-based rubber, and a styrene butadienerubber.

<13> A nonaqueous electrolyte secondary battery including a laminatedporous film recited in any one of the above <1> through <12>.

Advantageous Effects of Invention

With the present invention, it is possible to obtain a laminated porousfilm which has excellent ion permeability, which can maintain theexcellent ion permeability even when exposed under a high temperatureenvironment, and which is suitable as a nonaqueous electrolyte secondarybattery separator.

DESCRIPTION OF EMBODIMENTS

The following description will discuss the present invention in detail.The present invention is not limited to the embodiment, and can befreely altered in various ways by a person skilled in the art within thescope of the claims.

A laminated porous film of the present invention includes: a porous basematerial layer (hereinafter referred to also as “A layer”) containingpolyolefin as a main component; a filler layer (hereinafter referred toalso as “B layer”) containing inorganic particles as a main component;and a resin layer (hereinafter referred to also as “C layer”) containingparticular resin particles (hereinafter referred to simply as “resinparticles”) as a main component. The laminated porous film includes theA layer, the B layer, and the C layer, and the C layer containsparticular resin particles as a main component. This provides excellention permeability and maintains the excellent ion permeability even whenthe laminated porous film is exposed under a high temperatureenvironment.

Since the laminated porous film has excellent ion permeability even whenit is obtained as a result of drying at a high temperature, thelaminated porous film can be dried at a high temperature in a shorttime, resulting in excellent productivity. Furthermore, since thelaminated porous film maintains the excellent ion permeability even whenexposed under a high temperature environment in production of anonaqueous electrolyte secondary battery or used under a hightemperature environment, the laminated porous film is preferably usedfor a nonaqueous electrolyte secondary battery separator.

Note that the A layer melts so as to become non-porous in a case where abattery has generated intense heat. This provides a laminated porousfilm with a shutdown function. Note also that the B layer has heatresistance to withstand a high temperature where a shutdown occurs. Thisallows the laminated porous film, which includes the B layer, to haveshape stability even at a high temperature. Note also that, before athermal shrinkage temperature of the A layer is reached, the resinparticles of the C layer melt so as to cause the porous base materiallayer to be shaped into a non-porous film.

(Porous Base Material Layer (a Layer) Containing Polyolefin as MainComponent)

The A layer of the laminated porous film of the present invention willbe described below. The A layer has such an original function of aseparator as preventing a short circuit from occurring between a cathodeand an anode. The A layer can secure (i) a function as a support for theB layer and the C layer (described later) and (ii) a shutdown function.Examples of the shutdown function encompass a property to block ahole(s) of a separator at, for example, a temperature equal to or higherthan 80° C. (more preferably equal to or higher than 100° C.) and equalto or lower than 150° C. Specifically, in a case where a lithium ionsecondary battery of the present invention has reached a temperatureequal to or higher than a melting point (melting temperature measured byuse of a differential scanning calorimeter (DSC) according to JIS K7121) of polyolefin which is a main component of the A layer, thepolyolefin contained in the A layer melts so as to block a hole(s) of aseparator. This leads to a shutdown where advancement of anelectrochemical reaction is restricted.

The A layer is a porous layer containing polyolefin as a main component.Examples of the polyolefin encompass a high molecular weight homopolymerand a high molecular weight copolymer, each of which is obtained bypolymerizing, for example, ethylene, propylene, 1-butene,4-methyl-1-pentene, or 1-hexene. Any of these polyolefins can be usedindividually, or two or more of these polyolefins can be used incombination.

Among the polyolefins above, high molecular weight polyethylenecontaining ethylene as a main component is preferable.

In the present invention, what is meant by the “A layer containingpolyolefin as a main component” is that a percentage of polyolefincontained in the A layer relative to 100% by volume of constituents ofthe A layer is greater than 50% by volume. The percentage of apolyolefin component contained in the A layer relative to 100% by volumeof the constituents of the A layer is greater than 50% by volume,preferably equal to or greater than 70% by volume, more preferably equalto or greater than 90% by volume, and still more preferably equal to orgreater than 95% by volume.

Note that the A layer can contain a component in addition to polyolefin,provided that the function of the A layer is not impaired.

In a case where the A layer as a nonaqueous electrolyte secondarybattery separator is used for a nonaqueous electrolyte secondarybattery, it is preferable that a high molecular weight component havinga weight-average molecular weight of 1×10⁵ to 15×10⁶ is contained, andit is preferable that the weight-average molecular weight of thepolyolefin contained in the A layer falls within the above certainrange, in view of prevention of the A layer from being dissolved in anelectrolytic solution.

Porosity of the A layer is preferably 30% by volume to 80% by volume,and more preferably 40% by volume to 70% by volume. If the porosity isless than 30% by volume, then the separator may retain a small amount ofelectrolytic solution. If the porosity is greater than 80% by volume,then there is a risk that the A layer may be insufficiently non-porousat a high temperature where a shutdown occurs, that is, there is a riskof not being able to block an electric current in a case where a batterygenerates intense heat.

A thickness of the A layer is preferably 5 μm to 50 μm, and morepreferably 5 μm to 30 μm. If the thickness is less than 5 μm, then thereis a risk that the A layer may be insufficiently non-porous at a hightemperature where a shutdown occurs. If the thickness is greater than 50μm, then there is a risk that the laminated porous film may becomethick, and therefore a capacity of the battery may become small.

The inside of the A layer is structured so that pores are connected toeach other. This allows a gas, a liquid, or the like to pass through theA layer from one surface of the A layer to the other. The Airpermeability of the A layer is ordinarily 50 seconds/100 cc to 400seconds/100 cc, and preferably 50 seconds/100 cc to 300 seconds/100 cc,in terms of Gurley values. A pore diameter of the A layer is preferablyequal to or less than 3 μm, and still more preferably equal to or lessthan 1 μm.

A weight per unit area of the A layer is ordinarily 4 g/m² to 15 g/m²,and preferably 5 g/m² to 12 g/m². If the weight per unit area is lessthan 4 g/m², then there is a risk that strength of the laminated porousfilm becomes insufficient. If the weight per unit area is greater than15 g/m², then there is a risk that the laminated porous film may becomethick, and therefore a capacity of the battery may become small.

A method of producing the A layer is not limited to any particular one.Examples of the method encompass (A) a method in which (i) a plasticizeris added to polyolefin to shape the polyolefin into a film and then (ii)the plasticizer is removed with the use of a proper solvent (see, forexample, Japanese Patent Application Publication, Tokukaihei, No.7-29563) and (B) a method in which a structurally-weak amorphous portionof a film, which is produced by a well-known method and which isconstituted by polyolefin, is selectively drawn so as to form fine pores(see, for example, Japanese Patent Application Publication, Tokukaihei,No. 7-304110). For example, assume a case where the A layer containspolyolefin containing (i) ultra-high molecular weight polyethylene and(ii) low molecular weight polyolefin having a weight-average molecularweight of equal to or less than 10,000. In this case, in view ofproduction costs, the A layer is preferably produced by a method such asone described below. The ultra-high molecular weight polyolefin ispreferably a polyolefin having a weight-average molecular weight ofgreater than 1,000,000.

Specifically, the method to be employed is a method including the stepsof:

(1) kneading (i) 100 parts by weight of the ultra-high molecular weightpolyethylene, (ii) 5 parts by weight to 200 parts by weight of the lowmolecular weight polyolefin having a weight-average molecular weight ofequal to or less than 10,000, and (iii) 100 parts by weight to 400 partsby weight of an inorganic filler made of calcium carbonate and the like,to produce a polyolefin resin composition;(2) shaping the polyolefin resin composition into a sheet;(3) removing the inorganic filler from the sheet produced in the step(2); and(4) drawing the sheet obtained in the step (3), ora method including the steps of:(1) kneading (i) 100 parts by weight of the ultra-high molecular weightpolyethylene, (ii) 5 parts by weight to 200 parts by weight of the lowmolecular weight polyolefin having a weight-average molecular weight ofequal to or less than 10,000, and (iii) 100 parts by weight to 400 partsby weight of an inorganic filler, to produce a polyolefin resincomposition;(2) shaping the polyolefin resin composition into a sheet;(3) drawing the sheet obtained in the step (2) to obtain a stretchedsheet; and(4) removing the inorganic filler from the sheet produced in the step(3).

Note that the A layer can alternatively be a commercial product havingthe characteristics above.

(Filler Layer (B Layer) Containing Inorganic Particles as MainComponent)

The B layer of the laminated porous film of the present invention willbe described next. The B layer is a porous layer containing inorganicparticles as a main component. Since the B layer is a porous layercontaining inorganic particles as a main component, (i) it is possiblethat a gas, a liquid, or the like passes through the B layer from onesurface of the B layer to the other and (ii) it is possible to providethe laminated porous film with shape stability at a high temperature.

In the present invention, what is meant by the “B layer containinginorganic particles as a main component” is that a percentage ofinorganic particles contained in the B layer relative to 100% by weightof the constituents of the B layer is greater than 50% by weight. Thepercentage of the inorganic particles contained in the B layer relativeto 100% by weight of the constituents of the B layer is greater than 50%by weight, preferably equal to or greater than 70% by weight, preferablyequal to or greater than 90% by weight, and still more preferably equalto or greater than 95% by weight.

Examples of the inorganic particles encompass calcium carbonate, talc,clay, kaolin, silica, hydrotalcite, diatomaceous earth, magnesiumcarbonate, barium carbonate, calcium sulfate, magnesium sulfate, bariumsulfate, aluminum hydroxide, magnesium hydroxide, calcium oxide,magnesium oxide, titania, boehmite, alumina, mica, zeolite, and glass.Preferable examples of a material for the inorganic particles encompassalumina, boehmite, silica, and titania. Among these, alumina is morepreferable. The alumina is preferably α-alumina. These materials for theinorganic particles can be used individually, or two or more of theseinorganic particles can be used in a mixed state.

An average particle diameter of each inorganic particle is ordinarilyless than 3 μm, and preferably less than 1 μm. Each inorganic particleis not limited to any particular shape. Examples of a suitable shape ofthe inorganic particle encompass a plate-like shape, a granular shape,and a fibrous shape.

The B layer can contain a component in addition to the inorganicparticles, provided that the function of the B layer is not impaired.For example, the B layer can contain an organic binder.

The organic binder is ordinarily a polymer. It is preferable that (i)the polymer has a characteristic to bind the inorganic particlestogether and to bind the A layer and the inorganic particles together,(ii) the polymer is insoluble in an electrolytic solution of thebattery, and (iii) the polymer is electrochemically stable when thebattery is in normal use. While the organic binder can be awater-soluble polymer or a water-insoluble polymer, the organic binderis preferably a water-soluble polymer in view an environmental impactand of production costs. Examples of the water-soluble polymer encompasspolyvinyl alcohol, polyethylene glycol, cellulose ether, sodiumalginate, polyacrylic acid, polyacrylamide, and polymethacrylic acid.Among these, cellulose ether, polyvinyl alcohol, and sodium alginate arepreferable, and cellulose ether is still more preferable. These organicbinders can be used individually, or two or more of these organicbinders can be used in a mixed state.

Examples of the cellulose ether encompass carboxyalkyl cellulose, alkylcellulose, and hydroxyalkyl cellulose. Specific examples of thecellulose ether encompass carboxymethyl cellulose (CMC), hydroxyethylcellulose (HEC), carboxy ethyl cellulose, methyl cellulose, ethylcellulose, cyan ethyl cellulose, and oxyethyl cellulose. Among these,CMC is most preferable because deterioration of CMC after use for anextended period of time is little.

Alternatively, the cellulose ether can be a salt. Examples of the saltof CMC encompass a metal salt of CMC. The metal salt of CMC is excellentin maintaining its shape when heated. Sodium CMC, in particular, isversatile and can be obtained easily, and is therefore more preferable.

In a case where the B layer contains inorganic particles and an organicbinder, a weight proportion of the inorganic particles relative to 1part by weight of the organic binder is ordinarily 1 part by weight to100 parts by weight, and preferably 10 parts by weight to 50 parts byweight. In a case where the weight proportion of the inorganic particlesfalls within the above specified range, it is possible to obtain a Blayer having excellent strength while ion permeability is maintained.

Note that the B layer can contain a component in addition to inorganicparticles and an organic binder. Examples of such a component encompassa dispersing agent, a plasticizer, and a pH adjusting agent.

A thickness of the B layer is preferably 0.1 μm to 15 μm, and morepreferably 0.5 μm to 10 μm. If the thickness is less than 1 μm, thenthere is a risk that the B layer may fail to resist thermal shrinkage ofthe A layer in a case where the battery has generated intense heat, sothat shrinkage of the laminated porous film may occur. If the thicknessis greater than 15 μm, then there is a risk that an outputcharacteristic of a nonaqueous electrolyte secondary battery to beproduced may deteriorate.

A pore diameter of the B layer, as a diameter of each pore whose shapeis approximated as a sphere, is preferably equal to or less than 3 μm,and more preferably equal to or less than 1 μm. If an average diameterof the pores or a diameter of any pore is greater than 3 μm, then thereis a risk that, for example, a short circuit may easily occur in a casewhere a carbon powder, which serves as main components of a cathode andan anode, or a piece of the carbon powder falls off. Porosity of the Blayer is preferably 30% by volume to 70% by volume, and more preferably40% by volume to 60% by volume.

(Resin Layer (C Layer) Containing Resin Particles as a Main Component)

The C layer of the laminated porous film of the present invention willbe described next. The C layer is a porous layer containing resinparticles as a main component. The resin particles show an endothermiccurve satisfying conditions (1) and (2) below, the endothermic curvebeing obtained by differential scanning calorimetry.

Condition (1): a temperature at which DDSC of the endothermic curve isnot less than 0.10 mW/min/mg is not less than 70° C.Condition (2): endothermic amount calculated from an area of theendothermic curve in a range of not less than 50° C. and not more than70° C. is not less than −20.0 J/g.

DDSC is a value obtained by differentiating, with a time (min), a valueof a heat flow (mW) obtained by differential scanning calorimetry, anddividing the value thus obtained with a weight (mg) of the resin. DDSCindicates a change ratio.

Since the C layer contains the resin particles as a main component, aproper amount of void is maintained in the C layer. This causes anonaqueous electrolyte secondary battery, which includes the laminatedporous film having the C layer, to have reduced battery resistance. Thiscauses an output characteristic of the nonaqueous electrolyte secondarybattery to be excellent. Note that the C layer has a shutdown function.Particularly, in a case where the A layer contains, as a main component,high-melting polyolefin such as polypropylene, the shutdown function ofthe C layer is more effective. In the present invention, what is meantby the “C layer containing resin particles as a main component” is thata percentage of the resin particles contained relative to 100% by weightof the constituents of the C layer is greater than 50% by weight. Inorder to cause the shutdown function to be more effective, thepercentage of the resin particles contained in the C layer relative to100% by weight of the constituents of the C layer is preferably equal toor greater than 70% by weight, more preferably equal to or greater than80% by weight, and still more preferably equal to or greater than 90% byweight.

A temperature at which DDSC of the endothermic curve of the resinparticles, which endothermic curve is obtained by differential scanningcalorimetry, is not less than 0.10 mW/min/mg is not less than 70° C. Ina case where the temperature at which DDSC of the endothermic curve isnot less than 0.10 mW/min/mg is less than 70° C., air permeability ofthe laminated porous film is decreased. This is considered to be becausethe resin particles are dissolved in the drying step in production ofthe C layer and so the C layer reduces its pores, resulting in decreasein air permeability of the laminated porous film. Decrease in airpermeability of the laminated porous film leads to an increase inbattery resistance of a nonaqueous electrolyte secondary battery whichincludes the laminated porous film. This results in a decrease in outputcharacteristic of the nonaqueous electrolyte secondary battery.Furthermore, in the case where the temperature at which DDSC of theendothermic curve is not less than 0.10 mW/min/mg is less than 70° C.,air permeability of the laminated porous film in production of thenonaqueous electrolyte secondary battery is more likely to be decreased,and besides, air permeability of the laminated porous film when thenonaqueous electrolyte secondary battery is used under a hightemperature environment is more likely to be decreased.

Furthermore, in the endothermic curve obtained by differential scanningcalorimetry on the resin particles, an endothermic amount calculatedfrom the area of a melting curve at a range of not less than 50° C. andnot more than 70° C. is not less than −20 J/g. In a case whereendothermic amount is less than −20 J/g, air permeability of thelaminated porous film is decreased. This is considered to be because theresin particles are dissolved in the drying step in formation of the Clayer on the A layer and so the C layer reduces its pores, resulting indecrease in air permeability of the laminated porous film. Decrease inair permeability of the laminated porous film leads to an increase inbattery resistance of a nonaqueous electrolyte secondary battery whichincludes the laminated porous film. This may result in a decrease inoutput characteristic of the nonaqueous electrolyte secondary battery.Furthermore, in the case where endothermic amount is less than −20 J/g,air permeability of the laminated porous film in production of thenonaqueous electrolyte secondary battery is more likely to be decreased,and besides, air permeability of the laminated porous film when thenonaqueous electrolyte secondary battery is used under a hightemperature environment is more likely to be decreased. Endothermicamount is preferably not less than −15 J/g and not more than 0 J/g, morepreferably not less than −10 J/g and not more than 0 J/g, still morepreferably not less than −5 J/g and not more than 0 J/g, andparticularly preferably 0 J/g.

Differential scanning calorimetry on the resin particles may be madewith a temperature rising rate of 10° C./min. Furthermore, endothermicamount in a range of not less than ° C. and not more than 70° C. in anendothermic curve obtained by differential scanning calorimetry can becalculated according to JIS K 7122.

Examples of the resin particles encompass Fischer-Tropsch wax,low-density polyethylene (LDPE), low molecular weight polyethylene, andionomer. Among them, Fischer-Tropsch wax is preferable because it isinexpensive and has a linear chain structure and so exhibits physicalproperties suitable for the purpose of the present invention.Furthermore, Fischer-Tropsch wax having been fractionally distilled toremove a low-melting point component content is more preferable. Thesematerials of the resin particles can be used individually, or two ormore of these materials of the resin particles can be used in a mixedstate.

Note that the C layer can contain a component in addition to the resinparticles, provided that the function of the C layer is not impaired.Examples of the component encompass an organic binder.

The organic binder is ordinarily a polymer. It is preferable that (i)the polymer has a characteristic to bind the resin particles togetherand to bind the A layer and the resin particles together, (ii) thepolymer is insoluble in an electrolytic solution of the battery, and(iii) the polymer is electrochemically stable when the battery is innormal use. While the organic binder can be a water-soluble polymer or awater-insoluble polymer, the organic binder is preferably awater-insoluble polymer in view of a property to bond to the resinparticles. Examples of the water-insoluble polymer encompass astyrene-vinyl acetate copolymer, an ethylene-acrylic acid estercopolymer, a fluorine-based rubber, and a styrene-butadiene rubber.Among these, a styrene-butadiene rubber is preferable. These organicbinders can be used individually, or two or more of these organicbinders can be used in a mixed state.

In a case where the C layer contains resin particles and an organicbinder, a weight proportion of the resin particles relative to 1 part byweight of the organic binder is ordinarily 1 part by weight to 100 partsby weight, and preferably 10 parts by weight to 50 parts by weight. In acase where the weight proportion of the resin particles falls within theabove specified range, it is possible to obtain a C layer havingexcellent strength while ion permeability is maintained.

For the sake of, for example, increasing strength and an oxidizingproperty, the C layer can contain, in addition to the resin particlesand the organic binder, inorganic particles similar to those containedin the B layer described above, provided that the shutdown function isnot impaired. Alternatively, the C layer can contain a component,examples of which encompass a dispersing agent, a plasticizer, a pHadjusting agent, and a surfactant. Examples of the surfactant encompassan anionic surfactant and a nonionic surfactant. Among these, an anionicsurfactant is preferable in view of a property to enhance a shutdownfunction.

(Laminated Porous Film)

The laminated porous film of the present invention preferably includesthe A layer, the B layer, and the C layer, and is preferably configured,in consideration of shape stability at a high temperature, so that (i)the B layer is provided on one surface of the A layer and (ii) the Clayer is provided on the other surface of the A layer.

The laminated porous film is preferably configured so that a ratio ofthe weight per unit area of the B layer to the weight per unit area ofthe A layer (weight per unit area (g/m²) of B layer/weight per unit area(g/m²) of A layer) is 0.2 to 3.0. In a case where the ratio of theweight per unit area of the B layer to the weight per unit area of the Alayer falls within the above range, it is possible to maintain excellentair permeability while providing high heat-resistance.

In addition, the laminated porous film is preferably configured so thata ratio of the weight per unit area of the C layer to the weight perunit area of the A layer (weight per unit area (g/m²) of C layer/weightper unit area (g/m²) of A layer) is 0.1 to 2.0. In a case where theratio of the weight per unit area of the C layer to the weight per unitarea of the A layer falls within the above range, it is possible tomaintain excellent air permeability while providing a high shutdowncharacteristic.

In a case where the ratio of the weight per unit area of the B layer tothe weight per unit area of the A layer and the ratio of the weight perunit area of the C layer to the weight per unit area of the A layer fallwithin the above respective certain ranges, it is possible to obtain alaminated porous film having high safety and an excellent outputcharacteristic.

A thickness of the entire laminated porous film (A layer+B layer+Clayer) is ordinarily 5 μm to 75 μm, and preferably 10 μm to 50 μm. Ifthe thickness of the entire laminated porous film is less than 5 μm,then there is a risk that the laminated porous film may easily break. Ifthe thickness of the entire laminated porous film is greater than 75 μm,then there is a risk that the laminated porous film may become thick,and therefore a capacity of the battery may become small.

Air permeability of the laminated porous film is preferably 50 sec/100cc to 900 sec/100 cc. If the air permeability is greater than 900sec/100 cc, then battery characteristics (ion permeability, loadcharacteristics) may deteriorate.

As long as the object of the present invention is attained, thelaminated porous film of the present invention can contain a porouslayer in addition to the A layer, the B layer, and the C layer. Examplesof the porous layer encompass an adhesive layer and a protection layer.

A method of producing the laminated porous film will be described next.Examples of the method of producing the laminated porous film encompass(I) a method in which an A layer, a B layer, and a C layer areindividually produced and are then laminated together and (II) a methodin which (i) a B layer is formed by coating one surface of an A layerwith a coating solution which contains inorganic particles as a maincomponent and (ii) a C layer is formed by coating the other surface ofthe A layer with a coating solution which contains resin particles as amain component. Among these methods, the method (II) is easier and istherefore preferable.

Examples of the method (II) encompass a method including the followingsteps:

(1) coating an A layer with a slurry which contains inorganic particles,an organic binder, and a medium (B layer-forming slurry) so as to obtaina coating film and removing the medium from the coating film; and(2) coating the A layer with a slurry which contains resin particles, anorganic binder, and a medium (C layer-forming slurry) so as to obtain acoating film and removing the medium from the coating film.

Note that the coating film refers to a film provided on the A layer bycoating. By removing the medium from the coating film, a B layer and a Clayer are obtained, so that the B layer and the C layer are laminated onthe A layer. The order in which to carry out the steps (1) and (2) isnot particularly limited.

The slurry used in the above method can be obtained by, for example, amethod in which (i) an organic binder is dissolved or swelled in amedium (a liquid in which an organic binder is swelled may be used, ifthe liquid can be used for coating) and then (ii) inorganic particles orresin particles are added to the medium and mixed until a resultantmixture is uniform. A mixing method is not limited to any particularone, and can be carried out with the use of a conventionally knowndispersing device, examples of which encompass a three-one motor, ahomogenizer, a medium type dispersing device, and a pressure typedispersing device. A mixing order is not particularly limited, providedthat there arises no particular problem such as generation of aprecipitate.

The inorganic particles and the organic binder contained in the Blayer-forming slurry can be identical to the above described inorganicparticles and the organic binder which are contained in the B layer. Themedium only needs to allow the inorganic particles to be disperseduniformly and stably. Specific examples of the medium encompass: water;alcohols such as methanol, ethanol, and isopropanol; acetone; toluene;xylene; hexane; N-methylpyrrolidone; N,N-dimethylacetamide; andN,N-dimethylformamide. These media can be used individually, or two ormore of these media can be used in a mixed state, provided that the twoor more of these media are compatible. Among these, in view of a processand an environmental impact, it is preferable that equal to or greaterthan 80% by weight of the medium is water, and it is more preferablethat only water is used as the medium.

The resin particles and the organic binder contained in the Clayer-forming slurry can be identical to the above described resinparticles and the organic binder which are contained in the C layer. Theresin particles can be an aqueous emulsion obtained by dispersing, inwater, resin particles identical to the above described resin particlescontained in the C layer. The aqueous emulsion preferably contains asurfactant in view of improvement in storage stability. Examples of thesurfactant encompass those listed as examples of the surfactant whichcan be contained in the C layer. Among such surfactants, an anionicsurfactant is preferable. In a case where an aqueous emulsion containingresin particles and a surfactant is used as the resin particles, the Clayer to be obtained will contain the surfactant. In a case where thesurfactant is an anionic surfactant, the shutdown function of the Clayer to be obtained will improve. The medium only needs to allow theresin particles to be dispersed uniformly and stably. Specific examplesof the medium encompass: water; alcohols such as methanol, ethanol, andisopropanol; acetone; toluene; xylene; hexane; N-methylpyrrolidone;N,N-dimethylacetamide; and N,N-dimethylformamide. These media can beused individually, or two or more of these media can be used in a mixedstate, provided that the two or more of these media are compatible.Among these, in view of a process and an environmental impact, it ispreferable that equal to or greater than 80% by weight of the medium iswater, and it is more preferable that only water is used as the medium.

As long as the object of the present invention is attained, it ispossible to add, for example, a surfactant, a pH adjuster, a dispersingagent, and/or a plasticizer to the slurry. In a case where a surfactantis added to the slurry, it is possible to improve storage stability ofthe slurry. Examples of the surfactant encompass those listed asexamples of the surfactant which can be contained in the above describedC layer or in the aqueous emulsion. In a case where a surfactant isadded to the C layer-forming slurry, the C layer to be obtained willcontain the surfactant.

An organic binder concentration in the B layer-forming slurry relativeto 100% by weight of the organic binder and the medium combined isordinarily 0.2% by weight to 3.0% by weight, and preferably 0.2% byweight to 2.5% by weight. If the organic binder concentration is lessthan 0.2% by weight, then adhesion between the inorganic particles andadhesion between the A layer and the B layer at an interface between theA layer and the B layer may become lowered. This poses a risk that thecoating film may be peeled off, and that the B layer therefore cannot beprovided on the A layer so as to form a continuous membrane. If theorganic binder concentration is greater than 3.0% by weight, then alaminated porous film to be obtained may have impaired air permeability.Note that it is possible to adjust a molecular weight or the like of theorganic binder as necessary for obtaining slurry viscosity that issuitable to coating.

A solid content concentration in the B layer-forming slurry ispreferably 6% by weight to 50% by weight, and more preferably 9% byweight to 40% by weight. If the solid content concentration is less than6% by weight, then it may become difficult to remove the medium from theslurry. If the solid content concentration is greater than 50% byweight, then the B layer can easily become thick, and it may thereforebecome necessary, in order to form a B layer having a desired thickness,to coat the A layer with the slurry thinly.

An organic binder concentration in the C layer-forming slurry relativeto 100% by weight of the organic binder and the medium combined isordinarily 0.2% by weight to 3.0% by weight, and preferably 0.2% byweight to 2.5% by weight. If the organic binder concentration is lessthan 0.2% by weight, then adhesion between the resin particles andadhesion between the A layer and the C layer at an interface between theA layer and the C layer may become lowered. This poses a risk that thecoating film may be peeled off, and that the C layer therefore cannot beprovided on the A layer so as to form a continuous membrane. If theorganic binder concentration is greater than 3.0% by weight, then alaminated porous film to be obtained may have impaired air permeability.Note that it is possible to adjust a molecular weight or the like of theorganic binder as necessary for obtaining slurry viscosity that issuitable to coating.

A solid content concentration in the C layer-forming slurry ispreferably 6% by weight to 50% by weight, and more preferably 9% byweight to 40% by weight. If the solid content concentration is less than6% by weight, then it may become difficult to remove the medium from theslurry. If the solid content concentration is greater than 50% byweight, then the C layer can easily become thick, and it may thereforebecome necessary, in order to form a C layer having a desired thickness,to coat the A layer with the slurry thinly.

A method of coating the A layer with the slurry can be a conventionallywell-known method, and is not limited to any particular one, providedthat the method allows uniform wet coating. Examples of the methodencompass a capillary coating method, a spin coating method, a slit diecoating method, a spray coating method, a roll coating method, a screenprinting method, a flexographic printing method, a bar coater method, agravure coater method, and a die coater method. A thickness of each ofthe B layer and the C layer to be formed can be controlled by adjusting(i) the amount of the slurry to be applied, (ii) the organic binderconcentration in the slurry, and (iii) the ratio of the weight of theinorganic particles or the resin particles to the weight of the organicbinder.

In a case where water is contained as a medium, it is preferable tosubject the A layer to a hydrophilization treatment in advance beforethe A layer is coated with the slurry. In a case where the A layer issubjected to a hydrophilization treatment, the A layer becomes morecoatable. This makes it possible to obtain a B layer and a C layer whichare more homogeneous. The hydrophilization treatment is effectiveparticularly in a case where the medium has a high water concentration.

The A layer can be subjected to a hydrophilization treatment through anymethod. Specific examples of the method encompass (i) a chemicaltreatment in which an acid, an alkali, or the like is used, (ii) acorona treatment, and (iii) a plasma treatment.

Note that the corona treatment has the following advantages: (i) the Alayer can be hydrophilized in a relatively short amount of time and (ii)the A layer can be made highly coatable because only part of thepolyolefin, which part is located in the vicinity of a surface of the Alayer, is modified by corona discharge, so that the inside of the Alayer does not change in property.

A medium is removed from a coating film typically by drying. The mediumcan be removed from the coating film by, for example, (i) preparing asolvent which dissolves the medium and does not dissolve an organicbinder, (ii) immersing the coating film in the solvent to replace themedium with the solvent so that the organic binder is precipitated,(iii) removing the medium, and (iv) removing the solvent by drying. Notethat in a case where the A layer is coated with the slurry, atemperature, at which the medium or the solvent is dried, is preferablya temperature that does not cause a decrease in air permeability of theA layer.

(Nonaqueous Electrolyte Secondary Battery)

The following description will discuss the nonaqueous electrolytesecondary battery of the present invention. The nonaqueous electrolytesecondary battery of the present invention includes, as a separator, thelaminated porous film of the present invention. The nonaqueouselectrolyte secondary battery includes: (i) a cathode, (ii) an anode,(iii) a separator sandwiched between respective surfaces of the cathodeand the anode, which surfaces face each other, and (iv) a nonaqueouselectrolyte. Constituent elements of the nonaqueous electrolytesecondary battery of the present invention will be described below withan example in which the battery is a nonaqueous electrolyte secondarybattery typified by a lithium ion secondary battery. However, thepresent invention is not limited to such an example.

A nonaqueous electrolyte can be, for example, a nonaqueous electrolyteobtained by dissolving a lithium salt in an organic solvent. Examples ofthe lithium salt encompass LiClO₄, LiPF₆, LiAsF₆, LiSbF₆, LiBF₄,LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃, Li₂B₁₀Cl₁₀, lower aliphaticcarboxylic acid lithium salt, and LiAlCl₄. These lithium salts can beused individually, or a mixture of two or more of these lithium saltscan be used. Among these, it is preferable to use at least onefluorine-containing lithium salt selected from the group consisting ofLiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, and LiC(CF₃SO₂)₃.

Examples of the nonaqueous electrolyte encompass: carbonates such aspropylene carbonate, ethylene carbonate, dimethyl carbonate, diethylcarbonate, ethyl methyl carbonate,4-trifluoromethyl-1,3-dioxolane-2-one, and 1,2-di(methoxycarbonyloxy)ethane; ethers such as 1,2-dimethoxyethane,1,3-dimethoxypropane, pentafluoropropyl methylether,2,2,3,3-tetrafluoropropyl difluoro methylether, tetrahydrofuran, and2-methyl tetrahydrofuran; esters such as methyl formate, methyl acetate,and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile;amides such as N,N-dimethylformamide and N,N-dim ethylacetamide;carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compoundssuch as sulfolane, dimethyl sulfoxide, and 1,3-propane sultone; andnonaqueous electrolytes each obtained by introducing a fluorine groupinto any of the these substances. Ordinarily, two or more of these areto be used in a mixed state.

It is preferable to use a nonaqueous electrolyte containing any of theabove carbonates, and it is still more preferable to use (i) a mixtureof a cyclic carbonate and an acyclic carbonate or (ii) a mixture of acyclic carbonate and any of the ethers. The mixture of the cycliccarbonate and the acyclic carbonate is preferably a mixture of anethylene carbonate, a dimethyl carbonate, and an ethyl methyl carbonatebecause such a mixture allows a wide operating temperature range, and isnot easily decomposed even in a case where a graphite material such asnatural graphite or artificial graphite is used as an anode activematerial.

A cathode ordinarily includes (i) a cathode mix containing a cathodeactive material, a conductive material, and a binding agent and (ii) acathode current collector supporting the cathode mix thereon. Thecathode mix can be supported by the cathode current collector through amethod, examples of which encompass (i) a pressure forming method and(ii) a method in which (a) an organic solvent is used to obtain acathode mix paste, (b) the cathode current collector is coated with thecathode mix paste, (c) the cathode mix paste is dried to obtain a sheet,and (d) the sheet is pressed, so that the cathode mix is firmly fixed tothe cathode current collector. Specifically, the cathode mix can (i)contain, as the cathode active material, a material capable of dopingand dedoping lithium ions, (ii) contain a carbonaceous material as theconductive material, and (iii) contain, as the binding agent, an agentcontaining a thermoplastic resin or the like. Examples of the cathodecurrent collector encompass electric conductors such as Al, Ni, andstainless steel. Among these, Al is preferable because Al can easily beprocessed into a thin film and is inexpensive. Examples of the materialcapable of doping and dedoping lithium ions encompass a lithium complexoxide containing at least one transition metal such as V, Mn, Fe, Co, orNi. Preferable examples of the lithium complex oxide encompass (i) alithium complex oxide having an α-NaFeO₂ structure such as lithiumnickelate and lithium cobaltate and (ii) a lithium complex oxide havinga spinel structure such as lithium manganese spinel. This is becausethese lithium complex oxides have high average discharge potentials.

The lithium complex oxide can further contain any of various metallicelements. In particular, the lithium complex oxide is preferably complexlithium nickelate containing at least one metallic element selected fromthe group consisting of Ti, V, Cr, Mn, Fe, Co, Cu, Ag, Mg, Al, Ga, In,and Sn in an amount of 0.1 mol % to 20 mol % relative to the sum of thenumber of moles of the at least one metallic element and the number ofmoles of Ni in the lithium nickelate. This is because such a complexlithium nickelate allows improvement in cycle characteristic of abattery in use at high load.

Examples of the binding agent encompass thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and athermoplastic polyimide, polyethylene, and polypropylene.

Examples of the conductive material encompass carbonaceous materialssuch as natural graphite, artificial graphite, cokes, and carbon black.These conductive materials can be used individually, or, for example,artificial graphite and carbon black can be used in a mixed state.

Examples of the anode encompass (i) a material capable of doping anddedoping lithium ions, (ii) a lithium metal, and (iii) a lithium alloy.Examples of the material capable of doping and dedoping lithium ionsencompass: carbonaceous materials such as natural graphite, artificialgraphite, cokes, carbon black, pyrolytic carbons, carbon fiber, and afired product of an organic polymer compound; and chalcogen compoundssuch as an oxide and a sulfide that dope and dedope lithium ions at anelectric potential lower than that for the cathode. Among the abovecarbonaceous materials, a carbonaceous material containing a graphitematerial such as natural graphite or artificial graphite as a maincomponent is preferable because such a carbonaceous material has highelectric potential flatness and low average discharge potential and cantherefore be combined with a cathode to achieve high energy density.

Examples of an anode current collector encompass Cu, Ni, and stainlesssteel. Among these, Cu is preferable because Cu is not easily alloyedwith lithium in the case of particularly a lithium ion secondary batteryand is easily processed into a thin film. An anode mix containing ananode active material can be supported by the anode current collectorthrough a method, examples of which encompass (i) a pressure formingmethod and (ii) a method in which (a) a solvent or the like is used toobtain an anode mix paste, (b) the anode current collector is coatedwith the anode mix paste, (c) the anode mix paste is dried to obtain asheet, and (d) the sheet is pressed, so that the anode mix is firmlyfixed to the anode current collector.

Note that the battery of the present invention is not limited to anyparticular shape, and can have any of a paper shape, a coin shape, acylindrical shape, and a prism shape.

The laminated porous film of the present invention can be suitably usedas a separator for a battery, particularly for a nonaqueous electrolytesecondary battery. A nonaqueous electrolyte secondary battery, whichincludes the laminated porous film of the present invention, has a highoutput characteristic. Even in a case where abnormal heat generation hasoccurred, the laminated porous film fulfills a shutdown function, sothat further heat generation can be restricted. Even in a case whereintense heat has been generated, shrinkage of the laminated porous filmis restricted, so that a cathode and an anode can be prevented fromcoming into contact with each other.

EXAMPLES

The following description will discuss the present invention in detail.Note, however, that the present invention is not limited to thisdescription.

Physical properties and the like of the resin particles and thelaminated porous film were measured by the following method:

(1) Differential Scanning Calorimetry on Resin Particles

Measurement was performed with use of DSC200 manufactured by SeikoInstruments, Inc. with a rising temperature of 10° C./min. From anendothermic curve obtained as a result of the measurement, a temperatureat which the value of DDSC of the endothermic curve was not less than0.10 mW/min/m (which temperature may be hereinafter referred to as“temperature α) was obtained, and from an area of the endothermic curvein a range of not less than 50° C. and not more than 70° C., endothermicamount (which may be hereinafter referred to as “endothermic amount β”was calculated.

(2) Weight Per Unit Area (Unit: g/m²) of A Layer

A sample, which had a square shape having sides of 0.08 m, was cut outfrom a polyethylene porous film. Then, the weight W (g) of the samplethus cut out was measured so as to calculate the weight per unit area(=W/(0.08×0.08)) of the A layer.

(3) Weight Per Unit Area (Unit: g/m²) of B Layer

The polyethylene porous film (A layer) was subjected to a coronatreatment. Then, one surface of the polyethylene porous film was coatedwith a B layer-forming slurry, and the B layer-forming slurry was thendried at 70° C. for 30 seconds. This produced a laminated film includingthe B layer provided on one surface of the A layer. A sample, which hada square shape having sides of 0.08 m, was cut out from the laminatedfilm thus produced. The weight W (g) of the sample thus cut out wasmeasured so as to calculate the weight per unit area (=W/(0.08×0.08)) ofthe laminated film. Then, the weight per unit area of the B layer wascalculated by subtracting the weight per unit area of the film beforecoating of the B layer (the weight per unit area of the A layer) fromthe weight per unit area of the laminated film thus calculated.

(4) Weight Per Unit Area (Unit: g/m²) of C Layer

The polyethylene porous film (A layer) was subjected to a coronatreatment. Then, one surface of the polyethylene porous film was coatedwith a C layer-forming slurry, and the C layer-forming slurry was thendried at 80° C. for 5 minutes. This produced a laminated film includingthe C layer provided on one surface of the A layer. A sample, which hada square shape having sides of 0.08 m, was cut out from the laminatedfilm thus produced. The weight W (g) of the sample thus cut out wasmeasured so as to calculate the weight per unit area (=W/(0.08×0.08)) ofthe laminated film. Then, the weight per unit area of the C layer wascalculated by subtracting the weight per unit area of the film beforecoating of the C layer (the weight per unit area of the A layer) fromthe weight per unit area of the laminated film thus calculated.

(5) Thickness (Unit: μm)

The thickness of the laminated porous film was measured with the use ofa high-resolution digital measuring device manufactured by MitutoyoCorporation.

(6) Air Permeability (Unit: sec/100 cc)

Air permeability of the laminated porous film was measured according toJIS P8117 with the use of a digital timer Gurley densometer manufacturedby TOYO SEIKI SEISAKU-SHO, LTD.

(7) Air Permeability after being Left Still Under High TemperatureEnvironment (Unit: sec/100 cc)

Initially, the laminated porous film was left still for 1 hour in anoven set to have a temperature of 80° C. Next, air permeability of thelaminated porous film taken out from the oven was measured according toJIS P8117 with the use of a digital timer Gurley densometer manufacturedby TOYO SEIKI SEISAKU-SHO, LTD.

(8) Film Resistance of Laminated Porous Film (α, Unit: Ω·cm²)

A measurement sample having a circular shape of 17 mm in diameter wascut out from the laminated porous film. Furthermore, members of a 2032type coin cell (top cover, bottom cover, gasket, spacer (circular SUSspacer of 15.5 mm in diameter and 0.5 mm in thickness)×2, and wavewasher) (purchased from Hohsen Corp.) were prepared.

Initially, in a glove box filled with an argon gas and set to have a dewpoint of not more than −80° C., one of the spacers, the measurementsample, the other of the spacers were put in this order on the bottomcover.

Then, the gasket was put in such a manner as to fix the circularmeasurement sample of 17 mm in diameter, and the wave washer was put onthe other of the spacer. Next, into the cell provided with the wavewasher, an electrolyte of 1M in concentration (manufactured by KishidaChemical Co., Ltd.) was poured. This electrolyte was obtained byblending LiPF₆ with a mixture solvent of ethylene carbonate (EC),dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC)(EC/DMC/EMC=30/35/35 [volume ratio]). After pouring the electrolyte, thecell was left still for 10 minutes at a pressure of approximately −80kPa, so that the measurement sample was immersed with the electrolyte.Thereafter, the cell was covered with the top cover, and sealed with acoin-cell caulker to obtain a sample cell.

The sample cell thus obtained was put in an oven and heated at a rate of15° C./min. Respective resistances of the cell at cell temperatures of90° C., 100° C., 120° C., and 140° C. were measured with use of analternating impedance measuring device with amplitude of 5 mV andfrequency of 10 kHz. Values obtained by multiplying the individualresistances with an actual measurement area not covered with the gasket(1.88 cm² (=(1.55 cm/2)²×π)) were regarded as film resistances (Ω·cm²)of the laminated porous film at the aforementioned temperatures,respectively.

The A layer, the B layer, and the C layer were formed by a porous layer,inorganic particles, resin particles, and organic binders shown below.

<A Layer>

Porous layer: commercially available polyethylene porous film(thickness: 12 μm, weight per unit area: 7.2 g/m², air permeability: 212sec/100 cc)

<B Layer>

Inorganic particles: commercially available α-alumina (“AKP3000”manufactured by Sumitomo Chemical Co., Ltd.)

Organic binder: commercially available sodium carboxymethyl cellulose(CMC) (“CMC1110” manufactured by Daicel Corporation)

<C Layer>

Resin Particles 1: Fischer-Tropsch wax (temperature α: 73° C.,endothermic amount β: 0.0 J/g, particle size: 1.0 μm (particle size wasmeasured by a laser diffraction method))

Resin Particles 2: commercially available polyethylene wax (temperatureα: 45° C., endothermic amount β: −23.8 J/g, particle size: 1.0 μm(particle size was measured by a Coulter counter method))

Resin Particles 3: commercially available polyethylene wax (temperatureα: 50° C., endothermic amount β: −25.3 J/g, particle size: 2.5 μm(particle size was measured by a Coulter counter method))

Resin Particles 4: commercially available polyethylene wax (temperatureα: 65° C., endothermic amount β: −50.2 J/g, particle size: 1.0 μm(particle size was measured by a Coulter counter method))

Organic binder: commercially available styrene-butadiene rubber (SBR)(“AL2001” manufactured by NIPPON A&L INC.)

Example 1 <Production of B Layer-Forming Slurry>

A mixture was obtained by mixing α-alumina, CMC, and water together sothat an amount of CMC was 3 parts by weight relative to 100 parts byweight of α-alumina and a solid content concentration (CMC+α-alumina)was 27.7% by weight. Then, the B layer-forming slurry was prepared byprocessing the mixed solution under high-pressure dispersion conditions(100 MPa×3 passes) by use of a high-pressure dispersing device (“StarBurst” manufactured by Sugino Machine Limited).

<Production of C Layer-Forming Slurry>

The C layer-forming slurry was prepared by mixing Resin Particles 1dispersed in water containing an anionic surfactant, SBR, water, andisopropyl alcohol so that (i) an amount of SBR was 3 parts by weightrelative to 100 parts by weight of a solid content of the resinparticles and a solid content concentration (SBR+resin particles) was30.0% by weight and (ii) a solvent had a composition of 80% by weight ofwater and 20% by weight of isopropyl alcohol.

<Production of Laminated Porous Film>

The laminated porous film, in which (a) the B layer is provided on onesurface of the A layer and (b) the C layer is provided on the othersurface of the A layer, was obtained by (i) coating, with the Blayer-forming slurry, the one surface of the polyethylene porous film (Alayer) which has been subjected to the corona treatment, (ii) drying theB layer-forming slurry at 70° C. for 30 seconds to form the B layer,(iii) coating the other surface of the A layer with the C layer-formingslurry, and (iv) drying the C layer-forming slurry at 80° C. for 5minutes to form the C layer. The results of evaluation of physicalproperties of the laminated porous film thus obtained are shown inTable 1. Furthermore, an increased amount of air permeability of theobtained laminated porous film left still under a high temperatureenvironment was calculated in accordance with an equation below. Theresult of the calculation is shown in Table 1. As the increased amountof air permeability is smaller, the laminated porous film can maintainlarger ion permeability even when the laminated porous film is exposedto a high temperature environment.

Increased amount of air permeability=air permeability after being leftstill under a high temperature environment−air permeability before beingleft still under a high temperature environment

Furthermore, film thickness of the obtained laminated porous film wasmeasured. The result of the measurement is shown in Table 1.

Comparative Example 1

In production of a C layer-forming slurry, a laminated porous film wasobtained by carrying out a process similar to that of Example 1 exceptthat Resin Particles 2 dispersed in water containing a nonionicsurfactant was used instead of Resin Particles 1 dispersed in watercontaining an anionic surfactant. The results of evaluation of physicalproperties of the laminated porous film thus obtained are shown inTable 1. Furthermore, in a manner similar to that of Example 1, anincreased amount of air permeability of the laminated porous filmbetween before and after being left still under a high temperatureenvironment was calculated. The result of the calculation is shown inTable 1. Furthermore, in a manner similar to that of Example 1, filmresistance of the laminated porous film thus obtained was measured. Theresult of the calculation is shown in Table 1.

Comparative Example 2

In production of a C layer-forming slurry, a laminated porous film wasobtained by carrying out a process similar to that of Example 1 exceptthat Resin Particles 3 dispersed in water containing no surfactant wasused instead of Resin Particles 1 dispersed in water containing ananionic surfactant. The results of evaluation of physical properties ofthe laminated porous film thus obtained are shown in Table 1.Furthermore, in a manner similar to that of Example 1, an increasedamount of air permeability of the laminated porous film between beforeand after being left still under a high temperature environment wascalculated. The result of the calculation is shown in Table 1.

Comparative Example 3

In production of a C layer-forming slurry, a laminated porous film wasobtained by carrying out a process similar to that of Example 1 exceptthat Resin Particles 4 dispersed in water containing no surfactant wasused instead of Resin Particles 1 dispersed in water containing ananionic surfactant. The results of evaluation of physical properties ofthe laminated porous film thus obtained are shown in Table 1.Furthermore, in a manner similar to that of Example 1, an increasedamount of air permeability of the laminated porous film between beforeand after being left still under a high temperature environment wascalculated. The result of the calculation is shown in Table 1.Furthermore, in a manner similar to that of Example 1, film resistanceof the laminated porous film thus obtained was measured. The result ofthe calculation is shown in Table 1.

TABLE 1 Increased air permeability after Surfactant Weight Weight Weightbeing contained per per per Thick- Thick- Thick- Air left still in Cunit unit unit ness ness ness Per- at layer- area of area of area of ofA of B of C meability 80° C./1 hr forming A layer B layer C layer layerlayer layer [sec/ [sec/ Film resistance [Ω · cm²] slurry [g/m²] [g/m²][g/m²] [μm] [μm] [μm] 100 cc] 100 cc] 90° C. 100° C. 120° C. 140° C. Ex.Anionic 7.2 10.0 6.6 11.9 5.9 9.9 581 0 3.9 × 10¹ 6.5 × 10¹ 2.7 × 10⁵4.4 × 10⁵ Com. Nonionic 7.2 10.0 7.4 11.9 5.9 7.9 2282 3250 3.7 × 10¹3.2 × 10² 7.2 × 10³ 2.5 × 10⁵ Ex. 1 Com. None 7.2 10.0 6.6 11.9 5.9 11.2977 1650 Ex. 2 Com. None 7.2 8.9 7.7 11.9 5.3 10.9 796 316 2.8 × 10¹ 3.6× 10² 7.8 × 10³ 3.6 × 10⁵ Ex. 3

INDUSTRIAL APPLICABILITY

With the present invention, it is possible to obtain (i) a laminatedporous film which has excellent ion permeability, which maintains theexcellent ion permeability even when it is exposed to a high temperatureenvironment, and which is suitable as a nonaqueous electrolyte secondarybattery separator and (ii) a nonaqueous electrolyte secondary batteryincluding the laminated porous film.

1. A laminated porous film comprising: a porous base material layercontaining polyolefin as a main component; a filler layer containinginorganic particles as a main component; and a resin layer containingresin particles as a main component and an anionic surfactant.
 2. Thelaminated porous film as set forth in claim 1, wherein the resinparticles are Fischer-Tropsch wax.
 3. The laminated porous film as setforth in claim 1, wherein: the filler layer is provided on one surfaceof the porous base material layer; and the resin layer is provided onthe other surface of the porous base material layer.
 4. The laminatedporous film as set forth in claim 1, wherein: a ratio of a weight perunit area of the filler layer to a weight per unit area of the porousbase material layer is 0.2 to 3.0; and a ratio of a weight per unit areaof the resin layer to a weight per unit area of the porous base materiallayer is 0.1 to 2.0.
 5. The laminated porous film as set forth in claim1, wherein the inorganic particles are at least one selected from thegroup consisting of alumina, boehmite, silica, and titania.
 6. Thelaminated porous film as set forth in claim 1, wherein the inorganicparticles are α-alumina.
 7. The laminated porous film as set forth inclaim 1, wherein the filler layer contains an organic binder.
 8. Thelaminated porous film as set forth in claim 7, wherein the organicbinder is a water-soluble polymer.
 9. The laminated porous film as setforth in claim 8, wherein the water-soluble polymer is at least oneselected from the group consisting of carboxyalkyl cellulose, alkylcellulose, and hydroxyalkyl cellulose.
 10. The laminated porous film asset forth in claim 1, wherein the resin layer contains an organicbinder.
 11. The laminated porous film as set forth in claim 10, whereinthe organic binder is a water-insoluble polymer.
 12. The laminatedporous film as set forth in claim 11, wherein the water-insolublepolymer is at least one selected from the group consisting of anethylene-vinyl acetate copolymer, an ethylene-acrylic acid estercopolymer, a fluorine-based rubber, and a styrene butadiene rubber. 13.A nonaqueous electrolyte secondary battery comprising: a laminatedporous film recited in claim 1.